How to give my armature skeleton a body?
The approach is reversed. You need a model first, then an rig/armature to put into the mesh, then bind the mesh to the armature and then pose it.
Beside it looks like you posed some bones at the legs in the wrong direction as they point upwards.
Start with a basic tutorial to get the basic knowledge:
https://www.youtube.com/watch?v=f2pTkW-1JkE